To determine the number of calls you need the telephone system to handle at once, start with at least one call for each employee who uses the phone full-time -- customer service, support, and phone salespeople -- and add 1-2 extra calls for some callers to be on hold. For general office staff, one call for every 10 employees is a good estimate.
This question is essentially a tally of the number of users the system will have. Each individual user or common area phone will require a unique "extension." Extensions are required for each device that connects to the business phone system. Allow one for every employee’s desk phone, as well as one per fax machine, credit card terminal, modem, and any other equipment that requires a phone connection.
Please describe in detail any additional requirements you may have for this business phone system.